1. Field of the Invention
This invention relates to a key telephone system (hereunder briefly referred to as a "KT system").
2. Description of the Prior Art
A KT system is essential for making a connection between one of a plurality of telephone sets and a selected one of a plurality of central office (CO) lines extending to the KT system. A prior art KT system includes a plurality of telephone sets and a central unit adapted to selectively set up a connection between the telephone sets and the CO lines. Each of the telephone sets is provided with a hold button and a plurality of sets of CO lamps and CO buttons, each set corresponding on a one-to-one basis to the CO lines accommodated in the central unit. The CO lamps shows the user of the KT system a status of individually associated CO lines. A CO lamp turns off while the CO line is idle, turns on while the CO line is in use, flashes slowly while the CO line is being held, and flashes rapidly while an incoming call is being received over the CO line.
The CO buttons are available for selecting and seizing any of the CO lines. The central units detect an operation of the CO buttons which may occur at any telephone set so as to connect the telephone set to the selected CO line. For example, to make an outgoing call, the CO button is operated to seize the idle CO line. To answer an incoming call, the CO button is operated to seize the CO line over which the incoming call is being received. Further, while a certain CO line which has been connected to a certain telephone set is being held by the hold button, it may be picked up at another telephone set through the operation of its CO button associated with that CO line. Stated another way, the CO line over which an incoming call is received may be transferred from one telephone set to another.
An example of a KT line of the type being described is disclosed in U.S. Pat. No. 4,132,860. The problem with such a prior art KT system is that the number of the CO lamps and of the CO buttons on each station set have to be increased in proportion to the number of the CO lines, rendering the telephone set bulky and expensive.
In the above-described prior are KT system, to pick up an idle CO line for initiating an outgoing call or to seize a CO line for answering an incoming call which is received over that CO line, the user at the telephone set needs to check the CO lamps and then depress one of the CO buttons which is associated with the CO line. A KT system which, for a more efficient operation, allows the telephone set to automatically seize a predetermined one of the CO lines as a prime line when a handset thereof is taken up is described in U.S. Pat. No. 4,196,316. This automatic acquisition scheme, however, still has a problem while the prime line is in use, since it is necessary to select and seize another CO line, by means of an awkward operation. Especially, concerning an incoming call which usually requires a rapid response, a quick recognization of the CO line having an incoming call is so difficult that the user is apt to interrupt a circuit which is in use due to an inadvertent operation of the CO buttons. Such erroneous operation of the CO buttons, also occurs frequently while a circuit is in a hold condition as a result of a misperception of the flashing conditions of the CO lamps.